JP4083718B2 - Electro-optic sensor - Google Patents

Description

  The present invention relates to a highly sensitive electro-optic sensor capable of obtaining an output electric signal having a large amplitude.

  An EO sensor using an electro-optic (hereinafter referred to as EO as appropriate) crystal is widely used as a voltage sensor or the like, and its importance is increasing year by year.

The EO sensor utilizes the fact that the birefringence of the EO crystal changes due to the coupling of the electric field, so that light is incident on the EO crystal whose birefringence has changed in this way, the phase of the light is changed, and the light emitted from the EO crystal. The phase change amount is detected electrically.
Japanese Patent No. 3507007 Japanese Patent No. 3507008 JP 2003-98205 A

  However, the amount of phase change of the light that has passed through the EO crystal once is small, so that it is difficult to obtain an output electric signal having a large amplitude, that is, to increase sensitivity.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a highly sensitive electro-optic sensor capable of obtaining an output electric signal having a large amplitude.

In order to solve the above problems, the present invention of claim 1 is directed to an electro-optic crystal whose refractive index changes by coupling of an electric field, an electrode for coupling an electric field to the electro-optic crystal, and the electro-optic crystal. A light source for allowing light to enter; a film formed on an emission surface of light from the electro-optic crystal; or a mirror disposed behind the emission surface, which transmits and reflects light; and the electro-optic A film formed on the incident surface of light from the light source of the crystal or a mirror disposed in front of the incident surface, a second member that transmits and reflects light, and a first member that emits light from the first member to the back A first outgoing light detector for converting one outgoing light into an electrical signal; a polarizing beam splitter for taking out the second outgoing light emitted from the second member; and a second outgoing light taken out by the polarizing beam splitter as electric Second outgoing light detection to convert to signal And vessels, and solutions with an electro-optical sensor, characterized in that it comprises a differential amplifier for differentially amplifying the converted electrical signals are in the second outgoing light detector, the first outgoing light detector.

  According to the first aspect of the present invention, by providing the first member and the second member, the first member and the second member constitute a light resonator, and a large amount of light is contained in the electro-optic crystal. Since it reciprocates times, an output electric signal having a large amplitude can be obtained.

Further, according to the present invention as set forth in claim 1, a polarization beam splitter for taking out a second outgoing light emitted from the second member forward, first converting the second outgoing light taken out by the polarizing beam splitter into an electric signal 2 output light detectors, and a differential amplifier that differentially amplifies each electrical signal converted by the first output light detector and the second output light detector, thereby doubling the amplitude of the output signal Can be increased. Further, the DC component can be made zero by the differential signal detection.

According to the second aspect of the present invention, the reflectance of the resonator constituted by the first member and the second member when the electric field is not coupled to the electro-optic crystal has the largest change rate of the reflectance. and solutions with an electro-optical sensor according to claim 1 Symbol mounting, characterized in that the reflectance when.

According to the second aspect of the present invention, since the reflectance is the reflectance when the change rate of the reflectance is the largest, the light incident on the first outgoing light detector and the second outgoing light detector respectively. Can be made equal, thereby significantly reducing laser intensity noise during differential signal detection. It is also possible to prevent the amplifier from being saturated.

The present invention of claim 3 includes means for detecting a direct current component from the output of the differential amplifier and changing the frequency of light from the light source or the position of at least one of the mirrors so that the direct current component is eliminated. The electro-optical sensor according to claim 1 or 2 is used as a solving means.

The present invention of claim 3 includes means for detecting a direct current component from the output of the differential amplifier and changing the frequency of light from the light source or the position of at least one mirror so that the direct current component is eliminated. It is possible to prevent drift of the output signal due to temperature change.

According to a fourth aspect of the present invention, the surface of the first member with respect to the second member and the surface of the second member with respect to the first member are concave spherical surfaces, and the back of the first member and the first member claims 1, characterized in that a convex lens and in front of the two members with an electro-optical sensor according to any one of 3 to solutions.

According to this invention of Claim 4 , the surface with respect to the 2nd member of a 1st member and the surface with respect to the 1st member of a 2nd member are made into a concave spherical surface, The back of a 1st member, and the front of a 2nd member Since the convex lenses are arranged at both sides, it is possible to prevent the amplitude of the output electric signal from being lowered due to the diffusion of light within the electro-optic crystal.

  According to the electro-optic sensor of the present invention, since the first member and the second member are provided with the first member and the second member, an output electrical signal having a large amplitude is obtained. It is possible to provide a highly sensitive electro-optic sensor.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First embodiment]
FIG. 1 is a configuration diagram of an EO sensor 1 according to the first embodiment of the present invention.

  The EO sensor 1 includes an EO crystal 11 whose refractive index changes due to electric field coupling, electrodes 12 and 13 for coupling an electric field to the EO crystal 11, and a light source 14 that makes light such as laser light incident on the EO crystal 11. A film 11a (first member) formed on the light emission surface from the EO crystal 11, and a film 11b (second member) formed on the light incident surface of the EO crystal 11 from the light source 14. It comprises a photodetector PD1 (first emitted light detector) that converts light (first emitted light) emitted from the film 11a to the back into an electrical signal. Each film is formed so as to transmit and reflect light, that is, transmit a part of the light and reflect a part thereof by, for example, High-Refrection Coating: HRC using a dielectric multilayer film.

  The electrode 12 is connected to the measured signal input port PIN. The electrode 13 is connected to an object having a reference potential or placed in a floating state. The output terminal of the photodetector PD is connected to the output port POUT.

  In the EO sensor 1, the films 11a and 11b constitute an optical resonator (hereinafter simply referred to as a resonator).

  FIG. 2A shows the phase difference generated when light reciprocates in the resonator when no electric field is coupled to the EO crystal 11 on the horizontal axis, and the reflectivity (hereinafter simply referred to as reflection of the resonator). 2) is a graph in which a portion where the phase difference is in the vicinity of 2π is enlarged. The transmittance graph of the resonator is obtained by inverting the top and bottom of the reflectance graph.

  As shown in these figures, the reflectance of the resonator is, for example, the largest change rate when the phase difference is slightly smaller than the resonance point where the phase difference is 2π, and the point where the change rate is largest is the operating point. As best. The value is about 0.5. For example, the reflectance can be obtained by setting the intensity reflectance of the film 11a and the film 11b to about 80%. Note that the reflectivity of the resonator may be, for example, not less than 0.3 and not more than 0.7 for convenience of manufacturing. In the following description, the reflectance is assumed to be 0.5, but the present invention is not necessarily implemented with the reflectance being limited.

  In the EO sensor 1, when light from the light source 14 enters the resonator, reflection is repeated in the resonator, and about half of the incident light is emitted from the resonator. Half of the light is emitted to the photodetector PD1 side, and the other half is emitted to the light source 14 side.

  When a signal to be measured is applied to the signal to be measured input port PIN in this state, an electric field is coupled to the EO crystal 11, and the phase difference when light reciprocates in the resonator changes. For example, the detector PD1 side The amount of light emitted to the light changes. At this time, since the resonator is constituted by the film, and the reflectance of the resonator is changed abruptly by setting the reflectance of the resonator as described above, the light detector PD1 side is brought to this side. The amount of change in the emitted light is much larger than when no resonator is configured. Accordingly, an output electrical signal corresponding to the signal under measurement and having a large amplitude is output from the photodetector PD to the output port POUT.

  Therefore, according to the first embodiment, the films are provided on the light incident surface and the light exit surface of the EO crystal 11 so that each film constitutes a resonator (light resonator). Since the light travels back and forth many times, an output electric signal having a large amplitude can be obtained. Further, by setting the reflectance of the resonator as described above, an output electric signal having a larger amplitude can be obtained.

[Second Embodiment]
FIG. 3 is a configuration diagram of the EO sensor 2 according to the second embodiment of the present invention.

  The EO sensor 2 includes, in addition to the configuration of the EO sensor 1, an optical circulator 15 including a polarizing beam splitter PBS1 that extracts light (second outgoing light) emitted from the film 11b to the front, and light extracted by the polarizing beam splitter PBS1. A photodetector PD2 (second outgoing light detector) for converting the signal into an electrical signal, a differential amplifier DA for differentially amplifying the electrical signals converted by the photodetector PD1 and the photodetector PD2, and the differential A low-pass filter LPF that passes a low-frequency component (referred to as a DC component for convenience) and an integrator 16 from the output of the amplifier DA is provided, and the frequency of light from the light source 14 is adjusted by the output of the integrator 16 so that the DC component is eliminated. It is supposed to change.

  Now, in the EO sensor 2, the linearly polarized light from the light source 14 is separated into S-polarized light and P-polarized light by the polarization beam splitter PBS1, and the P-polarized light is the Faraday rotator FR, the half-wave plate HWP, the polarized beam of the optical circulator 15. The light passes through the splitter PBS2 sequentially and enters the resonator.

  When P-polarized light enters the resonator, reflection is repeated in the resonator, and P-polarized light having an amount that is approximately half of the incident P-polarized light is emitted from the resonator. Half of the light is emitted to the photodetector PD1 side, and the other half is emitted to the light source 14 side.

  When a signal under measurement including an AC component is applied to the signal under test input port PIN in this state, an electric field is coupled to the EO crystal 11. As a result, the phase difference when light reciprocates in the resonator changes, so that the transmittance and reflectance of the resonator change in opposite phases. That is, the reflectance decreases at the moment when the transmittance of the resonator increases, and the reflectance increases at the moment when the transmittance decreases.

  The photodetector PD1 converts the P-polarized light emitted to the photodetector PD1 side into an electric signal. On the other hand, the P-polarized light emitted to the light source 14 side passes through the polarization beam splitter PBS2, and further passes through the half-wave plate HWP and the Faraday rotator FR in order to become S-polarized light. The polarization beam splitter PBS1 reflects this S-polarized light to the photodetector PD2. This S-polarized light is converted into an electric signal by the photodetector PD2.

  The differential amplifier DA differentially amplifies the electric signals converted by the photodetectors PD1 and PD2. Since the transmittance and reflectance of the resonator are equal, the direct current components of the electrical signals output from the photodetectors PD1 and PD2 are equal. Therefore, a direct current component is not included in the output signal of the differential amplifier DA. Further, by differential signal detection, it is possible to obtain a signal amplitude that is twice that of the first embodiment. In addition, laser intensity noise can be reduced. Therefore, detection with higher sensitivity is possible.

  By the way, in the EO sensor 2, unless the frequency of the light from the light source 14 is appropriately determined, the transmittance and reflectance of the resonator cannot be made equal. In other words, when the signal under measurement is zero, the output of the differential amplifier cannot be zero. Even if the frequency of light is set appropriately, the frequency often varies depending on the ambient temperature, so that the output of the differential amplifier drifts.

  Therefore, in the EO sensor 2, the frequency of the light from the light source 14 is controlled by the low pass filter LPF and the integrator 16 so that the DC component of the output signal of the differential amplifier is eliminated (becomes zero). As a result, the output drift of the differential amplifier due to the ambient temperature change or the like can be suppressed, so that the signal under measurement can always be detected stably and with high sensitivity.

[Third embodiment]
FIG. 4 is a configuration diagram of the EO sensor 3 according to the third embodiment of the present invention.

  In the EO sensor 3, since the quarter-wave plate QWP is provided between the optical circulator 15 and the resonator in the EO sensor 2, the light reflected by the resonator becomes circularly polarized light and enters the optical circulator 15. .

  The incident light is separated into S-polarized light and P-polarized light by the optical circulator 15, and converted into electric signals by the photodetectors PD21 and PD2. Therefore, the output of the adding amplifier AA2 corresponds to the total intensity of the light reflected by the resonator.

  On the other hand, the light transmitted through the resonator is separated into S-polarized light and P-polarized light by the polarization beam splitter PBS3, and then converted into electric signals by the photodetectors PD11 and PD1, respectively. Therefore, the output of the adding amplifier AA1 corresponds to the total intensity transmitted through the resonator.

  When a signal under measurement is applied to the EO crystal 11, the S-polarized light and P-polarized light components change in opposite phases according to the signal, so that a signal corresponding to the signal under measurement is sent from the differential amplifier DA1 to the output port POUT. Is output.

  Since the output of the differential amplifier DA2 corresponds to the difference in intensity between the light transmitted through the resonator and the reflected light, the DC component of the output of the differential amplifier DA2 is the same as in the second embodiment. By controlling the frequency of light using the LPF and the integrator 16 so as to eliminate this, the intensity ratio between the transmitted light and the reflected light of the resonator can always be kept at 1: 1.

[Fourth embodiment]
FIG. 5 is a configuration diagram of the EO sensor 4 according to the fourth embodiment of the present invention.

  The EO sensor 4 is provided with a mirror 17a (first member) that transmits and reflects light similarly to the above in the back of the light emission surface of the EO crystal 11 instead of the film 11a of the EO sensor 2. In place of the film 11b, a similar mirror 17b (second member) is provided in front of the light incident surface of the EO crystal 11, and the DC component in the output of the differential amplifier DA is detected by the low-pass filter LPF and the integrator 16. The position of the mirror 17a (position in the direction of the optical path) is changed so that this direct current component is eliminated.

  The direct current component in the output of the differential amplifier DA can be kept constant by controlling the optical path length in the resonator, not the frequency of light. In the EO sensor 4, the low-pass filter LPF and the integrator 16 detect a DC component in the output of the differential amplifier DA, and change the position of the mirror 17a. In the EO sensor 4, the position of the mirror 17b may be changed.

  Therefore, according to the fourth embodiment, the DC component in the output of the differential amplifier DA is detected, and the position of the mirror is changed so that this DC component is eliminated. Drift due to can be prevented.

[Fifth embodiment]
FIG. 6 is a configuration diagram of the EO sensor 5 according to the fifth embodiment of the present invention.

  In the EO sensor 5, the surface of the mirror 17a with respect to the mirror 17b is a concave spherical surface, the surface of the mirror 17b with respect to the mirror 17a is a concave spherical surface, and a convex lens 18a is disposed behind the mirror 17a. A convex lens 18b is arranged in front of 17b.

  In the EO sensors 1 to 4 described so far, when the reflectance of the film or mirror is high, or when the loss of light in the EO crystal 11 is small, the Q value of the resonator increases, and the EO crystal 11 The number of reflections within the EO increases, and light diffuses within the EO crystal 11. When the distance between the electrodes 12 and 13 is short, reflection of light at the electrodes may occur and the output electric signal may become unstable.

  In the EO sensor 5, the light from the light source 14 side can be narrowed down within the EO crystal 11 by providing the convex lens 18 b. Further, the narrowly focused light spreads in the EO crystal 11 and is emitted to the photodetector PD1, but the convex lens 18a is provided to prevent the light to the photodetector PD1 from remaining spread. be able to.

  In the EO sensor 5, the surface of the mirror 17a with respect to the mirror 17b is a concave spherical surface, and the surface of the mirror 17b with respect to the mirror 17a is a concave spherical surface, so that the light spreading from the EO crystal 11 is reflected. In addition, the reflected light can be narrowed down and returned to the EO crystal 11.

[Sixth embodiment]
FIG. 7 is a configuration diagram of the EO sensor 6 according to the sixth embodiment of the present invention.

  In the EO sensor 6, the surface of the film 11a with respect to the film 11b is a concave spherical surface, the surface of the film 11b with respect to the film 11a is a concave spherical surface, and a convex lens 18a is disposed behind the mirror 11a. A convex lens 18b is arranged in front of 11b.

  In the sixth embodiment, since the film of the EO sensor 6 acts in the same manner as the mirror of the EO sensor 5, the same effect as the EO sensor 5 can be obtained.

  In the EO sensor 5 and the EO sensor 6, the light focused by the convex lens is not a plane wave but its wavefront is a spherical surface due to a diffraction effect. Therefore, the radius of curvature of the concave spherical surface and the radius of curvature of the wavefront of the light are as follows. Is preferably matched.

  Therefore, according to the fifth or sixth embodiment, the surface of the membrane or mirror with respect to the other side is a concave spherical surface, and the convex lenses are arranged in front of and behind the membrane or mirror. It is possible to prevent the amplitude of the output electric signal from the differential amplifier DA from being reduced due to light diffusion.

[Seventh embodiment]
FIG. 8 is a configuration diagram when the EO sensor 2 is used for human body communication.

  When the EO sensor 2 is used for human body communication, since the EO sensor 2 is often carried, the electrode 13 is connected to the ground electrode 13a of the EO sensor 2 that is away from the ground. The measured signal input port PIN connected to one of the electrodes 12 is connected via a lead wire to the receiving electrode 51 brought into contact with the human body. Further, the transmission electrode 52 is in contact with the human body, and the transmitter 53 is connected between the transmission electrode 52 and the ground.

  In the seventh embodiment, the signal of the transmitter 53 is given as an input signal to the measured signal input port PIN via the transmission electrode 52 and the reception electrode 51. The EO sensor 2 outputs an output signal corresponding to the signal given to the measured signal input port PIN by the above-described operation. Moreover, as described above, an output electric signal having a large amplitude and little saturation can be obtained. In addition, an output electric signal corresponding to the AC component of the electric field coupled to the EO crystal 11 can be obtained. Further, it is possible to prevent a decrease in amplitude of the output electric signal and a drift due to temperature.

[Eighth embodiment]
FIG. 9 is a configuration diagram when the EO sensor 2 is used for inspection of a device under measurement.

  An input signal is given to the device under measurement 60 from a signal source 61. The device under measurement 60 is, for example, an apparatus that generates unnecessary electromagnetic waves. When inspecting such a device under measurement 60, the electronic components that constitute the EO sensor 2 are separated from the device under measurement. preferable.

  In the eighth embodiment, the EO crystal 11 of the EO sensor 2, the electrodes 12 and 13, and the optical circulator 15 are provided in a sensor head 100 including a metal needle 101 that is brought into contact with a measurement point of a device under measurement 60. It is done.

  On the other hand, the light source 14, the photodetectors PD1 and PD2, the differential amplifier DA, the low-pass filter LPF, and the integrator 16 are connected to the sensor head 100 by an optical fiber 301 that is a polarization maintaining fiber, a single mode. The optical fiber 302 is a single-mode fiber or a multi-mode fiber, and the signal processing apparatus 200 is connected by an optical fiber 303 that is a single-mode fiber or a multi-mode fiber.

  In the sensor head 100, the metal needle 101 is connected to the signal input port PIN to be measured connected to the electrode 12 via a lead wire, and the electrode 13 is connected to the ground electrode 102 of the sensor head 100.

  In the sensor head 100, a collimator 104, a collimator 105, and a collimator 106 are connected to respective end portions of the optical fiber 301, the optical fiber 302, and the optical fiber 303, and the collimator 104 is disposed in front of the optical circulator 15. .

  In the sensor head 100, the mirror 107 that reflects the light extracted by the optical circulator 15 and enters the collimator 105, and the two mirrors that sequentially reflect the light emitted to the back of the EO crystal 11 and enter the collimator 106. 107 and 108 are provided.

  On the other hand, in the signal processing device 200, the light source 14, the photodetector PD2, and the photodetector PD1 are disposed at the respective ends of the optical fiber 301, the optical fiber 302, and the optical fiber 303.

  In the eighth embodiment, when the metal needle 101 is brought into contact with the measurement point of the device under measurement 60, an input signal from the signal source 61 is applied to the signal input port PIN to be measured via the metal needle 101. . The light from the light source 14 is supplied to the optical circulator 15 through the optical fiber 301. The light from the film 11a is reflected by the mirrors 108 and 109, and is supplied to the photodetector PD1 through the optical fiber 303. The light from the film 11b is extracted by the optical circulator 15, reflected by the mirror 107, and supplied to the photodetector PD2 by the optical fiber 302. The other operation is the same as that of the second embodiment, and therefore, an output electric signal having a large amplitude and less saturation is obtained. In addition, an output electric signal corresponding to the AC component of the electric field coupled to the EO crystal 11 can be obtained. Further, it is possible to prevent a decrease in amplitude of the output electric signal and a drift due to temperature.

  Further, according to the eighth embodiment, the optical fiber 301 for supplying the light from the light source 14 to the EO crystal 11, the optical fiber 303 for supplying the light from the film 11a to the photodetector PD1, and 303. And the optical fiber 302 for supplying light to the photodetector PD2 from the film 11b, the light source 14, the photodetector PD1 and the photodetector PD2 can be separated from the EO crystal 11, and thus the electrical The degree of freedom of the configuration of the optical sensor can be increased. Thereby, for example, the output of the differential amplifier DA can be prevented from being affected by electromagnetic waves.

  In the above description, the configuration and operation effect of the EO sensor 1 are described, and then the configuration and operation effect unique to each of the EO sensor 2 and later are described. The effects according to the configuration can be obtained.

It is a lineblock diagram of EO sensor 1 concerning a 1st embodiment of the present invention. 2A is a graph of the phase difference generated when light reciprocates in the resonator and the reflectance of the resonator, and FIG. 2B is an enlarged graph of a portion where the phase difference is in the vicinity of 2π. It is. It is a block diagram of EO sensor 2 concerning a 2nd embodiment of the present invention. It is a block diagram of the EO sensor 3 which concerns on the 3rd Embodiment of this invention. It is a block diagram of the EO sensor 4 which concerns on the 4th Embodiment of this invention. It is a block diagram of EO sensor 5 which concerns on the 5th Embodiment of this invention. It is a block diagram of EO sensor 6 concerning a 6th embodiment of the present invention. It is a block diagram at the time of using the EO sensor 2 for human body communication. It is a block diagram at the time of using the EO sensor 2 for a test | inspection of a to-be-measured device.

Explanation of symbols

1, 2, 3, 4, 5, 6 EO sensor 11 EO crystal 12, 13 Electrode 11a, 11b Film 14 Light source 15 Optical circulator 16 Integrator 17a, 17b Mirror 18a, 18b Convex lens DA Differential amplifier PBS1, PBS2 Polarizing beam splitter PD1, PD2 Photo detector LPF Low pass filter

Claims (4)

An electro-optic crystal whose refractive index changes due to electric field coupling;
An electrode for coupling an electric field to the electro-optic crystal;
A light source for making light incident on the electro-optic crystal;
A first member that transmits and reflects light, which is a film formed on a light exit surface of the electro-optic crystal or a mirror disposed behind the light exit surface;
A film formed on the incident surface of light from the light source of the electro-optic crystal or a second member that is disposed in front of the incident surface and transmits and reflects light;
A first outgoing light detector for converting the first outgoing light emitted from the first member into the back to an electrical signal ;
A polarization beam splitter for extracting second emitted light emitted from the second member to the front;
A second outgoing light detector for converting the second outgoing light extracted by the polarizing beam splitter into an electrical signal;
An electro-optic sensor comprising: a differential amplifier that differentially amplifies each electric signal converted by the first outgoing light detector and the second outgoing light detector . The reflectance of the resonator composed of the first member and the second member when the electric field is not coupled to the electro-optic crystal is the reflectance when the change rate of the reflectance is the largest. electro-optical sensor of claim 1 Symbol mounting features. Claim 1, wherein detecting a DC component from the output of the differential amplifier, characterized by comprising means for changing the position of the frequency or at least one of the mirrors of the light from the light source as the DC component is eliminated Or the electro-optic sensor of 2 . The surface of the first member with respect to the second member and the surface of the second member with respect to the first member are concave spherical surfaces,
Electro-optical sensor according to any of claims 1 to 3, characterized in that a convex lens and in front of the back and the second member of the first member.

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